The success of a cancer therapy generally depends on its ability to target certain unique requirements of the cancer cells that are distinct from those of the normal cells in the body. Current targeted cancer therapies at different stages of development mostly focus on inhibiting the deregulated oncogenic pathways. clinical studies of tyrosine kinase inhibitors such as Gefitinib and Gleevec (inhibitors of EGFR and Abl tyrosine kinases, respectively) revealed two general properties of these targeted cancer drugs: 1) they are generally only effective to a small subset of cancers with genetic alterations that make these cancers addicted to the deregulated oncogenic signaling being targeted; 2) cancers that are resistant to these inhibitors will eventually develop. These observations suggested the need to have large number of cancer drugs that target different features of cancer cells. In this case, different combinations of targeted cancer drugs can be used to specifically target different subsets of cancers and to prevent the development of resistant cancers. In addition to deregulated oncogenic activation, cancer cells also often have inactivation of tumor suppressor genes. However, very little effort has been devoted to develop therapeutic approaches that target such loss of tumor suppressor function in cancers. The main difficulty with targeting the loss of function tumor suppressors is the lack of straightforward approaches to either restore the lost tumor suppressor function in all the cancer cells or to specifically kill cancer cells with inactivated tumor suppressors. The Rb tumor suppressor is often inactivated in different types of cancers by mutation of the Rb gene itself, by loss of Rb expression, or by its functional inactivation. The Rb pathway is highly conserved in Drosophila. We have carried out a genetic screen in Drosophila to identify genes that can modulate the apoptosis of Rb mutant cells. Our genetic screen has led to the identification of a gene that is required specifically for preventing the apoptosis of Rb mutant cells both in developing fly tissues and in cancer cells. These observations suggest that our generic screen have led to the identification of a gene that can potentially be used as a target to specifically kill cancer cells with inactivated Rb tumor suppressor. In this grant, we will further investigate this idea to determine the mechanisms involved and to investigate the subset of cancer cells that can potentially be targeted. Furthermore we will develop assays to screen for small molecule inhibitors that can be used to specifically kill cancer cells with inactivated Rb pathway.